1. Field of the Invention
The present invention relates to a water jet propeller having an impeller disposed inside an impeller housing. In particular, the present invention relates to making the impeller rotate to expel a water jet.
2. Description of Background Art
Conventionally, a water jet propeller is mounted in a rear portion of a hull. The water jet propeller draws in water from a hull bottom by driving the impeller with an engine and expelling the water drawn therein rearward, thereby propelling a watercraft (see, for example, Japanese Patent Laid-open No. Hei 9-99897).
Japanese Patent Laid-open No. Hei 9-99897 will be described with reference to the FIG. 11. FIG. 11 is a view for illustrating a basic construction of a water jet propeller according to the background art.
A water jet propeller 200 includes an impeller housing 203, a stator 204, a nozzle 205 and a steering nozzle 207. The impeller housing 203 has a cylindrical shape and is disposed in a rear portion 202 of a hull 201. The stator 204 has a cylindrical shape and is disposed at a rear end portion 203a of the impeller housing 203. The nozzle 205 has a diameter that decreases rearwardly. The nozzle 205 is disposed at a rear end portion 204a of the stator 204. The steering nozzle 207 is pivotably mounted to the nozzle 205 via upper and lower pins 206, 206, so that the steering nozzle 207 can swing in a crosswise direction.
An impeller 208 is disposed inside the impeller housing 203. A rotational shaft 209 of the impeller 208 extends into the stator 204. The extended rotational shaft 209 is rotatably mounted in the stator 204 via bearings 211, 211.
In the water jet propeller 200 according to the background art, the impeller 208, which is disposed inside the impeller housing 203, is rotated by an engine (not shown). A water jet is thereby expelled from a rear end portion 207a of the steering nozzle 207 to propel the hull 201. The steering nozzle 207 can be swung to the right or to the left about an axis of the upper and lower pins 206, 206, thereby allowing the hull 201 to turn right or left.
A water jet propeller according to the background art includes a stainless steel liner applied to an entire area from a front end portion 203c to a rear end portion 203d of the inner periphery 203b of the impeller housing 203. The stainless steel liner offers outstanding wear resistance. Therefore, the stainless steel liner has been used in the background art as an example of a solution to decrease wear of an inner periphery 203b of the impeller housing 203 that can occur during rotation of the impeller 208.
However, applying a stainless steel liner to the entire area from the front end portion 203c to the rear end portion 203d of the inner periphery 203b of the impeller housing 203 causes the liner to have an inside diameter that is uniform throughout the entire area from the front end portion 203c to the rear end portion 203d. Accordingly, if, for example, the inner periphery 203b of the impeller housing 203 is made smaller than an inside diameter of a rear end portion 212a of a water flow duct 212, a step is formed between the rear end portion 212a of the water flow duct 212 and the front end portion 203c of the inner periphery 203b. 
The step formed between the rear end portion 212a of the water flow duct 212 and the front end portion 203c of the inner periphery 203b blocks the flow of water when there is an inflow of water from the water flow duct 212 into the impeller housing 203. Since the water flow is blocked off by the step, it is difficult to ensure an efficient flow of water into the impeller housing 203 from the water flow duct 212.